Primary (non-rechargeable) electrochemical cells having an anode of lithium are known and are in widespread commercial use. The anode is comprised essentially of lithium metal. Such cells typically have a cathode comprising manganese dioxide, and electrolyte comprising a lithium salt such as lithium trifluoromethane sulfonate (LiCF3SO3) dissolved in a nonaqueous solvent. The cells are referenced in the art as primary lithium cells (primary Li/MnO2 cells) and are generally not intended to be rechargeable. Alternative primary lithium cells with lithium metal anodes but having different cathodes, are also known. Such cells, for example, have cathodes comprising iron disulfide (FeS2) and are designated Li/FeS2 cells. The iron disulfide (FeS2) is also known as pyrite. The Li/MnO2 cells or Li/FeS2 cells are typically in the form of cylindrical cells, typically AA size or AAA size cells, but may be in other size cylindrical cells. The Li/MnO2 cells have a voltage of about 3.0 volts which is twice that of conventional Zn/MnO2 alkaline cells and also have higher energy density (watt-hrs per cm3 of cell volume) than that of alkaline cells. The Li/FeS2 cells have a voltage (fresh) of between about 1.2 and 1.8 volts which is about the same as a conventional Zn/MnO2 alkaline cell. However, the energy density (watt-hrs per cm3 of cell volume) of the Li/FeS2 cell is higher than a comparable size Zn/MnO2 alkaline cell. The theoretical specific capacity of lithium metal is high at 3861.7 mAmp-hr/gram and the theoretical specific capacity of FeS2 is 893.6 mAmp-hr/gram. The FeS2 theoretical capacity is based on a 4 electron transfer from 4Li per FeS2 molecule to result in reaction product of elemental iron Fe and 2Li2S. That is, 2 of the 4 electrons change the oxidation state (valence) of +2 for Fe+2 in FeS2 to 0 in elemental iron (Fe0) and the remaining 2 electrons change the oxidation state of sulfur from −1 in FeS2 to −2 in Li2S. In order to carry out the electrochemical reaction the lithium ions, Li+, produced at the anode must transport through the separator and electrolyte medium and to the cathode.
Overall the Li/FeS2 cell is much more powerful than the same size Zn/MnO2 alkaline cell. That is for a given continuous current drain, particularly at higher current drain over 200 milliAmp, the voltage drops off much less quickly for the Li/FeS2 cell than the Zn/MnO2 alkaline cell as may be evident in a voltage vs. time profile. This results in a higher energy output obtainable from a Li/FeS2 cell compared to that obtainable for a same size alkaline cell. The higher energy output of the Li/FeS2 cell is also clearly shown more directly in graphical plots of energy (Watt-hrs) versus continuous discharge at constant power (Watts) wherein fresh cells are discharged to completion at fixed continuous power outputs ranging from as little as 0.01 Watt to 5 Watt. In such tests the power drain is maintained at a constant continuous power output selected between 0.01 Watt and 5 Watt. (As the cell's voltage drops during discharge the load resistance is gradually decreased raising the current drain to maintain a fixed constant power output.) The graphical plot Energy (Watt-Hrs) versus Power Output (Watt) for the Li/FeS2 cell is above that for the same size alkaline cell. This is despite that the starting voltage of both cells (fresh) is about the same, namely, between about 1.2 and 1.8 volt.
Thus, the Li/FeS2 cell has the advantage over same size alkaline cells, for example, AAA, AA, C or D size or any other size cell in that the Li/FeS2 cell may be used interchangeably with the conventional Zn/MnO2 alkaline cell and will have greater service life, particularly for higher power demands. Similarly the Li/FeS2 cell which is a primary (nonrechargeable) cell can be used as a replacement for the same size rechargeable nickel metal hydride cell, which has about the same voltage (fresh) as the Li/FeS2 cell.
The Li/MnO2 cell and Li/FeS2 cell both desirably employ non aqueous electrolytes, since the lithium anode is highly reactive with water. One of the difficulties associated with the manufacture of a Li/FeS2 cell is the need to add good binding material to the cathode formulation to bind the Li/FeS2 and carbon particles together in the cathode. The binding material must also be sufficiently adhesive to cause the cathode coating to adhere uniformly and strongly to the conductive substrate to which it is applied.
The cathode material may be initially prepared in a form such as a slurry mixture, which can be readily coated onto the metal substrate by conventional coating methods. The electrolyte added to the cell is a suitable nonaqueous electrolyte for the Li/FeS2 system allowing the necessary electrochemical reactions to occur efficiently over the range of high power output desired. The electrolyte must exhibit good ionic conductivity and also be sufficiently stable, that is non reactive, with the undischarged or partially discharged electrode materials (anode and cathode components) and also non reactive with the discharge products. This is because undesirable oxidation/reduction reactions between the electrolyte and electrode materials (either discharged or undischarged or partially discharged) could gradually contaminate the electrolyte and reduce its effectiveness or result in excessive gassing. This in turn can result in a cell failure. Thus, the electrolyte used in a Li/FeS2 cell in addition to promoting the necessary electrochemical reactions, should also be stable in contact with discharged, partially discharged and undischarged electrode materials. Additionally, the electrolyte should enable good ionic mobility and transport of the lithium ion (Li+) from anode to cathode so that it can engage in the necessary reduction reaction resulting in Li2S product in the cathode.
Primary lithium cells are in use as a power source for digital flash cameras, which require operation at higher pulsed power demands than is supplied by individual alkaline cells. Primary lithium cells are conventionally formed of an electrode composite comprising an anode formed of a sheet of lithium (or lithium alloy, essentially of lithium), a cathode formed of a coating of cathode active material comprising FeS2 on a conductive metal substrate (cathode substrate) and a sheet of electrolyte permeable separator material therebetween. The electrode composite may be spirally wound and inserted into the cell casing, for examples, as shown in U.S. Pat. No. 4,707,421.
A cathode coating mixture for the Li/FeS2 cell is described in U.S. Pat. No. 6,849,360 B2 and U.S. Pat. No. 7,157,185 B2. The cathode described in these two references includes FeS2 particles, carbon particles (acetylene black and graphite), fumed silica, and a polymer binder preferably a styrene-ethylene/butylene-styrene (SEBS) block copolymer. Such binder is described as available as Kraton G1651 from Kraton Polymers, Houston Tex. These latter references describe that the cathode components are first made into a wet cathode slurry by adding solvent such as 1,1,2-trichloroethylene. The wet slurry is then applied to both sides of a carrier sheet, namely, a continuous aluminum strip, to form the wet cathode. It is implied that the wet cathode is then dried, since the phrase “after drying” appears (U.S. Pat. No. 6,849,360 at col. 6, line 3 and U.S. Pat. No. 7,157,185 at col. 6, line 33). There is no discussion in these two references of any specific manner in which the drying of the wet cathode is carried out. The references do not mention, nor are they concerned with, any particular drying method, drying atmosphere, or heating sequence and temperatures required to carry out the drying of the wet cathode. In fact there is no indication that any particular method of drying of the wet cathode or subsequent heat treatment of the dried cathode would be desirable or lead to better results.
A portion of the spiral wound anode sheet is typically electrically connected to the cell casing which forms the cell's negative terminal. The cell is closed with an end cap which is insulated from the casing. The cathode sheet can be electrically connected to the end cap which forms the cell's positive terminal. The casing is typically crimped over the peripheral edge of the end cap to seal the casing's open end. The cell may be fitted internally with a PTC (positive thermal coefficient) device or the like to shut down the cell in case the cell is exposed to abusive conditions such as short circuit discharge or overheating.
The anode in a Li/FeS2 cell can be formed by laminating a layer of lithium metal or lithium alloy on a metallic substrate such as copper. However, the anode may be formed of a sheet of lithium or lithium alloy without any substrate.
The electrolyte used in primary Li/FeS2 cells are formed of a “lithium salt” dissolved in an “organic solvent”. The electrolyte must promote ionization of the lithium salt and provide for good ionic mobility of the lithium ions so that the lithium ions may pass at good transport rate from anode to cathode through the separator. Representative lithium salts which may be used in electrolytes for Li/FeS2 primary cells are referenced in U.S. Pat. No. 5,290,414 and U.S. Pat. No. 6,849,360 B2 and include such salts as: Lithium trifluoromethanesulfonate, LiCF3SO3 (LiTFS); lithium bistrifluoromethylsulfonyl imide, Li(CF3SO2)2N(LiTFSI); lithium iodide, LiI; lithium bromide, LiBr; lithium tetrafluoroborate, LiBF4; lithium hexafluorophosphate, LiPF6; lithium hexafluoroarsenate, LiAsF6; Li(CF3SO2)3C; LiClO4; lithium bis(oxalato)borate, LiBOB and various mixtures. In the art of Li/FeS2 electrochemistry lithium salts are not always interchangeable as specific salts work best with specific electrolyte solvent mixtures.
In U.S. Pat. No. 5,290,414 (Marple) is reported use of a beneficial electrolyte for FeS2 cells, wherein the electrolyte comprises a lithium salt dissolved in a solvent comprising 1,3-dioxolane (DX) in admixture with a second solvent which is an acyclic (non cyclic) ether based solvent. The acyclic (non cyclic) ether based solvent as referenced may be dimethoxyethane (DME), ethyl glyme, diglyme and triglyme, with the preferred being 1,2-dimetoxyethane (DME). As given in the example the 1,2-dimethoxyethane (DME) is present in the electrolyte in substantial amount, i.e., at either 40 or 75 vol. % (col. 7, lines 47-54). A specific lithium salt ionizable in such solvent mixture(s), as given in the example, is lithium trifluoromethane sulfonate, LiCF3SO3. Another lithium salt, namely lithium bistrifluoromethylsulfonyl imide, Li(CF3SO2)2N is also mentioned at col. 7, line 18-19. The reference teaches that a third solvent may optionally be added selected from 3,5-dimethlyisoxazole (DMI), 3-methyl-2-oxazolidone, propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC), tetrahydrofuran (THF), diethyl carbonate (DEC), ethylene glycol sulfite (EGS), dioxane, dimethyl sulfate (DMS), and sulfolane (claim 19) with the preferred being 3,5-dimethylisoxazole.
In U.S. Pat. No. 6,849,360 B2 (Marple) is disclosed a specific preferred electrolyte for an Li/FeS2 cell, wherein the electrolyte comprises the salt lithium iodide dissolved in the organic solvent mixture comprising 1,3-dioxolane (DX), 1,2-dimethoxyethane (DME), and small amount of 3,5 dimethylisoxazole (DMI). (col. 6, lines 44-48) The electrolyte is typically added to the cell after the dry anode/cathode spiral with separator therebetween is inserted into the cell casing.
Contaminants can be introduced into the cell, from different sources, in particular, from the storage of FeS2 powder prior to its use in the cathode mix. It is stated: “Decomposition products resulting from the reaction of FeS2 with moisture are acidic in nature, and their introduction into the cell containing lithium is undesirable.” Jean-Paul Gabano (Ed.), Lithium Batteries, Academic Press (1983), p. 117 bottom. The stored FeS2 powder as well as cathodes based on FeS2 can gradually react with atmospheric air and moisture resulting in acidic and other byproducts, some capable of forming dendrites, which can all reduce cell life and can interfere with attainment of good cell performance during normal usage. For example, the dendrites can cause internal short circuiting of the cell and the acidic components may react with cell components such as the metallic current collectors. The acidic components may also induce polymerization of the electrolyte solvents. In published patent application US 2005/0277023 A1 (Marple) it is disclosed that a pH raising additive compound may be added to the FeS2 powder to counteract the acidic components. The reference indicates that adding a pH raising additive to the FeS2 can prevent or substantially prevent internal short circuits by reducing dendrite formation. (US 2005/0277023 A1, paragraph 90) Such pH raising additives recited in this reference include calcium oxide (CaO), calcium stearate, calcium hydroxide (Ca(OH)2), magnesium oxide (MgO), strontium oxide (SrO), and barium oxide (BaO). Overbased calcium sulfonates have a calcium carbonate (CaCO3) portion linked to the calcium sulfonate molecule. The carbonate portion is reported in the reference as responsible for raising the pH of the FeS2. Other pH raising additives for admixture with FeS2 or FeS2 cathodes disclosed in US 2005/0277023 A1 are organic amines such as diethylamine and triethylamine, cycloaliphatic epoxies such as butylenes oxide and soybean oil epoxide, and amino alcohols such as 2-amino-2-methyl-1-propanol.
Conventional FeS2 powders, for example Pyrox Red 325 powder from Chemetall GmbH, are commercially available with pH raising additives therein. (However, FeS2 powder can also be ordered without any pH raising additives.) Such additives are believed to contain calcium carbonate (CaCO3) or calcium carbonate linked to other compounds. Such calcium carbonate is added to the FeS2 powder to retard the formation of acidic impurities within or on the surface of the powder as it is stored in ambient air and exposed to oxygen and moisture present in air. Thus the calcium carbonate is conventionally added in this manner to reduce the buildup of acidic contaminants in the FeS2 powder. This is regardless of whether the FeS2 is intended for use in cathode mixtures or other applications, for example, as an additive in manufacture of car brakes.
The addition of pH raising additive such as calcium carbonate (CaCO3) or calcium carbonate containing compounds to the FeS2 powder, however, tends to cause agglomeration of the FeS2 particles when the FeS2 powder is stored in ambient air. Such agglomeration of the FeS2 powder can significantly interfere with attainment of the expected level of performance from Li/FeS2 cells. Also, the calcium carbonate or calcium carbonate containing compound additives has the disadvantage that such compounds get carried into the FeS2 cathode mixture. The calcium carbonate acts merely as an insulator within the cathode, that is, it is not electrochemically active and does not render the cathode more conductive. In other words the calcium carbonate takes up a certain amount of volume within the cathode that might otherwise be used for FeS2 active material. If calcium carbonate is admixed with FeS2 to raise pH it may typically comprise about 1.5 percent by weight of the mixture. However, calcium carbonate is less dense than FeS2. The bulk density ratio of FeS2 to CaCO3 is about 1.66. That is, 1.5 grams of calcium carbonate has the same volume as 2.49 grams FeS2. Thus for every 1.5 grams of calcium carbonate present in the cathode there are 2.49 grams less FeS2 active material that can be included in the cathode.
Accordingly, it is desired to reduce the rate of buildup of acidic contaminants in the FeS2 powder and thus in effect to extend the storage life of the FeS2 without sacrificing electrochemical cathode capacity. It is desired to reduce or eliminate altogether the amount of pH raising additive, such as calcium carbonate, that acts merely as an insulator and takes up volume within the cathode, which could otherwise be used for additional FeS2 active material.
Accordingly, it is desired to improve the storage life of FeS2 powder by adding to the powder a material other than calcium carbonate or other pH raising additive to retard buildup of acidic contaminants, wherein the new material unlike the calcium carbonate, is not merely and insulator within the cathode but also serves to improve electrochemical capacity or conductivity.
It is also desired to improve the method of forming the cathode for the Li/FeS2 cell, in particular to reduce the amount of acidic contaminants carried into the cell by the FeS2 powder.
It is desired that the method of treatment of the cathode reduces the amount of acidic contaminants therein before the cathode is inserted into the cell casing. It is desired that the treatment be such that it reduces the chance of the contaminants reoccurring.
It is desired to produce a primary (nonrechargeable) Li/FeS2 cell having good rate capability that the cell may be used in place of rechargeable batteries to power digital cameras.